To improve the calculation accuracy of design rainstorm in the process of evaluation of mountain flood disaster,the paper analyzed the variation characteristics of rainstorm-time-frequency with the long sequence data from lots of rainfall stations in Shandong province,devirated the law according to the principle of rainstorm formula in small river basin and put forward the multi-period conversation formula which is the optimization formula for the calculation of design rainstorm. The optimization formula accurately reflects the change rule of small watershed rainstorm and numerical characteristics and has more rationality and practicability.Taking the Licheng district of Jinan for example,the paper conducted the Study on the application of optimization formula.The result shows that the accuracy of optimization formula has improved 17.99% by an average compared to the current formula，which can provide the basis for the accuracy of design storm calculation and the analysis of early warning index.